Cranes have been an important part of industry for many years. Cranes driven by DC electric motors are used to hoist and move heavy loads from one location to another within the crane's service area. Since these loads can be extremely heavy, and can include molten metals in the iron and steel industries, it can easily be seen that an automatic braking system to stop the lowering of the load during a power outage is important. Two types of brakes, electrical and mechanical, are commonly used with electric motors. Electrical brakes are generally used when there is no tendency for rotation of the motor due to a heavy load. Dynamic braking is a common method of electrical braking used in DC motors. In dynamic braking, when power to motor circuit is removed, the motor will continue to rotate due to its momentum and will generate a counter electromotive force (CEMF) as long as it remains in rotation. Since the polarity of the CEMF is opposite to that of the voltage from the DC power supply, current flow in the armature will also be in the opposite direction. This reverse current flow in the motor circuit causes a torque opposite to the normal motor rotation to be developed, thus causing the motor slow. In dynamic braking, the speed at which the motor slows can be controlled by selectively changing the electrical resistance in either the field circuit, or the armature circuit of the motor. The most effective control is a combination of the two. In the field circuit, an acceleration resistor comprised of a bank of one or more resistors connected in series with the field coil are used to selectively weaken or strengthen the magnetic field through which the armature rotates. Low resistance in the field circuit produces a strong magnetic field and increases the CEMF produced while a high resistance in the field circuit weakens the magnetic field and decreases the CEMF produced. In the armature circuit, a dynamic braking resistor comprising one or more load resistors connected in the dynamic braking loop selectively controls current flow in the armature. Low resistance in the dynamic braking loop permits a high current flow in the armature and reduces the CEMF produced while a high resistance in the dynamic braking loop reduces current flow in the armature and increases the CEMF produced. The time required to bring the motor to a complete stop will depend on the resistance values of the acceleration resistor and the dynamic braking resistor in the motor circuit, the friction of the system, and the external load on the motor (weight of the load supported by a crane lifting motor). Dynamic braking is most effective in shunt or lightly compounded motors since the field is in parallel with the armature and therefore independent of armature current. During the lowering operation of a DC crane, the series wound DC motor is selectively manipulated into a shunt connected machine (motor, armature and field are connected in parallel) through the use of contacts controlled by a master switch in the crane control circuit. Configuring the motor as a shunt machine allows the motor to take advantage of dynamic braking when lowering a load. Mechanical brakes are generally spring-set brakes and are normally engaged when power to the motor is not present. When power is applied to the motor, the brake is released by means of a solenoid-operated mechanism that overcomes the force of the engagement spring. The solenoid is operated by a coil electrically connected in series with the motor, such that, when power is applied to the motor, the solenoid coil will be activated, thereby releasing the mechanical brake. When power is removed from the motor circuit, whether by normal crane operation or by a power outage, the solenoid coil is deactivated, thereby activating the spring-set mechanical brake. During normal crane operation, the master switch controls the direction and speed of the DC lifting motor by operating contacts in the motor circuit. When the master switch is moved to the OFF position, dynamic braking will quickly slow the motor to a stop. This will stop the generation of the CEMF and the flow of reverse current in the motor circuit thus deactivating the series-wound solenoid and activating the spring-set mechanical brake. However, if the crane is lowering a heavy load at some speed other than the lowest speed when power is lost, the weight of the load will cause the motor to continue rotating and thereby continue generating a CEMF. The reverse current in the motor circuit will prevent the setting of the mechanical brake by the series-wound solenoid. If the load is heavy enough to maintain the motor in an overhauling state, the dynamic brake can not slow the motor to a stop and the mechanical brake will not be set. This will result in the continued lowering of the load until it reaches the floor or other supporting means.
In an effort to overcome this problem, crane control systems have been provided with emergency stop switches and "dead man" switches in the crane operating circuitry. These switches would operate or control a contact in either the undervoltage circuit or the armature circuit, which would cause the armature circuit to be opened, thereby stopping the flow of current. The problem with these switches is that they require some action by the crane operator to initiate activation. During an emergency, this operator required action could be difficult or impossible. It is also possible for the switch operation to be defeated or rendered inoperable by the operator. This has been particularly true with respect to the "dead man" switch, which is generally a spring biased normally open switch requiring the operator to continuously hold it in the activated position while lowering the crane's load. A more recent method, as described in a paper by M. A. Urbassik entitled "Automatic Brake Setting During DCCP Regenerative Hoist Control Power Loss Condition", presented at the 1997 A.I.S.E. proceedings, employs a low voltage monitoring relay (LVMR) which monitors the DC bus voltage. The LVMR, upon sensing a change in the DC bus voltage, initiates the activation of the series brake. Although it is not disclosed how the series brake activation is initiated, it would be obvious to open a contactor in the undervoltage circuit of the crane control circuit or the motor circuit as in the "dead man" switch or E-Stop button. This particular application, as further described in the paper, requires some adjustment of the LVMR, depending on the characteristics of the particular crane system on which it is to be installed. It would therefore be desirable to have an automatic anti-regeneration circuit which can easily be connected to an existing crane control system, and which does not require any additional electrical adjustments for proper operation with the existing crane control system in which it is to be installed. It would also be desirable to have a completely electronic application of the automatic anti-regeneration circuit, thus eliminating mechanical elements such as contacts, which can have mechanical failures.